The present invention relates to a thermal printing apparatus for using a line thermal head.
In a conventional thermal printing apparatus for using a line thermal head, since a printing width is kept constant and a continuous thermal sensitive paper is used in correspondence with the printing width, the waste of the thermal sensitive paper sometimes increases according to a size of the original. To reduce this waste of the paper, various sizes of the separated papers may be used. However, there are the following problems. There has been no thermal printer having a line thermal head that can use several sizes of thermal sensitive papers.
In the line thermal head, in accordance with a content of the original or an output signal of the printing pattern, it is determined which element should be heated. Therefore, if a piece of cut paper is used and a size of the paper is selected in error, the width of printing pattern often becomes larger than expected. In this case, the heating elements which are located outside of the printing width of paper are also heated. Even if the size of paper is selected correctly, it is often happened that the width of the printing pattern is as long as the width of paper and the whole position of the printing pattern signal is slightly offset in the lateral direction In this case, the heating elements located outside of the paper width are also heated.
Thus, if the heating elements are heated without the thermal sensitive paper under the elements, the elements are heated over. Further, if the elements are heated over repeatedly, the elements are likely to be damaged or broken down. Some heating elements which are located near to the edges of the paper would be subjected to the above-mentioned "idle-heating". Further, if the heating elements which are located close to the edges of paper are repeatedly idle-heated in the thermal head, the heating elements are readily damaged to form a blank line on the printed surface in the longitudinal direction.
In this case, it is very difficult to replace only the broken and damaged elements of the line thermal head by new ones. Accordingly, the whole head must be exchanged by new head. Therefore, although the waste of paper is reduced, the cost of maintenance is increased. Also, in the case of using cut paper, before the leading edge of the paper passes under the thermal head or after the trailing edge of the paper passes under the thermal head, there would be fear that the printing signal would be inputted into a control means so that the heating elements would be accidentally subjected to idle heat.